1. Field of the Invention
The present invention relates to a data reproducing circuit, and more particularly, to a data reproducing circuit which records magnetization reversal patterns corresponding to written digital signals onto a magnetic recording medium, and converts received analog signals from the magnetic recording medium into digital signals.
2. Description of the Prior Art
The data reproducing circuit is broadly used for memory systems such as magnetic disc and magnetic drum devices. In recent years, improving recording density of magnetic media has substantially reduced intervals between magnetization reversals. The received analog signal peaks when a magnetization reversal occurs, but due to the influence of adjacent magnetization reversals, the peaks appear with certain time differences. This phenomenon is known as the peak shift. Accordingly, compensation of the peak shift is required in a data reproducing circuit. For higher record density in the future, the peak shift compensation is an important technique.
FIG. 26 is a principle view of the peak shift phenomenon. A magnetic recording medium stores binary data according to existence or non-existence of a magnetization reversal. For example, in data recording by 2-7 coding method, data to be recorded is converted into prescribed writing codes, and a magnetization reversal pattern corresponding to the writing code is recorded. The current applied to the magnetic head is reversed when the writing code is "1", which causes the magnetization direction of the magnetic recording medium to reverse so that a magnetization reversal pattern is recorded.
In the reproducing process, the received analog signal from the magnetic head should have the waveform shown with broken lines in relation to the magnetization reversal positions, but it actually shows the waveform shown with the solid line, which is obtained by integrating the broken line waveforms. Though the received analog signal should have peaks at magnetization reversals, the peaks appear with some deviation from the positions where the magnetization reversals occur, due to the influence of the adjacent magnetization reversals. This phenomenon is referred to as the "peak shift." In addition to the peak shift, peak level decline due to interference by waveforms is also observed.
The peaks shift to the side with a larger distance from the adjacent magnetization reversal. In terms of frequency of the magnetization reversals, a peak shifts to the side where the frequency is lower, when comparing the forward and backward frequencies. The larger the difference of the forward and backward frequencies is, the further the peak shifts.
FIG. 27 shows the relation between the frequencies of magnetization reversals and the peak level of the received analog signal. In the figure, f.sub.max indicates the maximum and f.sub.min indicates the minimum frequency of the magnetization reversal intervals. In 2-7 coding, two 1s, which represent magnetization reversals, sandwich two 0s for the maximum frequency, and 7 0s for the minimum frequency.
To solve the peak shift problem, some methods have been conventionally proposed as shown below.
One is the peak shift compensation in the recording process, which is disclosed in the Japanese Patent Application Laid-open Print No. 59-087610 and U.S. Pat. No. 4,607,295. By providing a shift equivalent to the expected peak shift amount in the opposite direction to that of the peak shift phenomenon when a magnetization reversal is recorded onto the magnetic recording medium, the peak shift is compensated so that the peak of the received analog signal can be obtained at the original timings when reproduced. This method requires a pre-shift circuit which analyzes the patterns of the codes to be recorded on the magnetic recording medium, and controls the recording process to provide a certain shift to the magnetization reversals.
The above peak shift compensation using the above pre-shift results in a higher recording frequency, i.e. narrowing intervals between magnetization reversals. Extremely narrow intervals between the magnetization reversals increases the interference by the waveform, and as shown in FIG. 27, lowers the peak level of the received analog signal from the magnetic head. Generally, when a received analog signal peak is higher than the predetermined slice level, a magnetization reversal is deemed to occur. Consequently, for a lower peak level, the slice level should be also determined lower. This may result in reduction in margins for noises observed at the reproducing circuit.
Another conventional method to solve the peak shift phenomenon is the peak shift compensation in the reproducing process, which is to obtain an equalized signal by removing the peak shifts from the received analog signal from the magnetic head using a reflection type cosine equalizer.
FIG. 28 shows the construction of a reflection type cosine equalizer in a conventional embodiment.
The reflection type cosine equalizer has a delay circuit 811 having an open output end, a gain adjustment circuit 821, and a differential amplifier 831 for subtracting an output from the gain adjustment circuit 821 from the output from the delay circuit 811. The input signal f(t) is delayed at the delay circuit 811 by a delay time .tau., and a delayed signal is supplied to a first input terminal of the amplifier 831. An input impedance of the amplifier 831 is very high, and the delayed signal supplied to the first input terminal is reflected to the delay circuit 811. The reflected signal is further delayed at the delay circuit 811 by the delay time .tau.. This further delayed signal is added to the gain adjustment circuit 821. The gain adjustment circuit 821 has a gain k, where k.ltoreq.1. Namely, the first input terminal of the amplifier 831 receives a signal f (t+.tau.), and the gain adjustment circuit 821 receives signals kf(t) and kf (t+2.tau.). As a result, as shown in FIG. 30, the differential amplifier 831 calculates a difference between an analog output f(t+.tau.) obtained by delaying the analog output by the delay time at the delay circuit 811 and an output k.multidot.f((t)+k.multidot.f(t+2.tau.) from the gain adjustment circuit 821, to equalize the analog output Sf into an analog output f'(t+.tau.) having a sharp waveform. This waveform has a cosine shape.
To completely compensate the peak shift by the above the input voltage ratio V-/V+ of the differential amplifier 831 at a high level to provide a large equalizing amount. A high gain k at the equalizer for a large equalization amount, however, amplifies the high frequency components according to the characteristics shown in FIG. 29. The amplified noise components of high range might be detected as excessive peaks during peak position detection by differentiation of the signal, which causes another peak shift problem. In addition, when the equalization amount is large, original waveform will have "shoulders" on both sides as shown in FIG. 31(b). The shoulders will be larger as the equalization amount becomes larger.
The output signal of the equalizer is used for peak position detection by differentiation, and for window signal generation by comparing it with the predetermined slice level. Accordingly, when the equalizing amount is set to a large value, the equalized signal will have large shoulders, and the noises on the shoulders might be higher than the slice level. This means that the peak shift compensation in the reproducing process also has a problem of noise margin reduction, which impedes a complete peak shift compensation.
Still another conventional method to solve the peak shift problem is to perform peak shift compensation in both recording and reproducing processes. A certain amount of pre-shift is provided in the recording process, and the remaining peak shift is compensated by the reflection type cosine equalizer in the reproducing process. In this method, the equalizing amount at the equalizer in the reproducing process should be set moderately, because the peak shift compensation is partly performed at the pre-shift circuit. Too small of an equalizing amount at the equalizer, however, cannot compensate the peak level decline due to high recording frequency, and the slice level for window signal generation cannot be set at a high level.
Writing codes include 2-7 coding and 1-7 coding methods. In 2-7 coding, a pair of 1s sandwiches at least two 0s and at most seven 0s, while in 1-7 coding, a pair of 1s sandwiches at least one 0 and at most seven 0s between them. For 2-7 coding, 1 data bit corresponds to 2 code bits, and for 1-7 coding, 2 data bits correspond to 3 code bits. When the transfer time of 1 data bit is supposed to be T, the reading time of 1 code bit is T/2 for 2-7 coding and 4T/3 for 1-7 coding. The maximum cycle of the magnetization reversal is 8T/2 for 2-7 coding and 16T/3 for 1-7 coding. Particularly in 1-7 coding, the wide recording frequency band greatly lowers the peak level at a high recording frequency.
The peak shift phenomenon above necessarily occurs for every magnetic head, regardless of the head type. For easier understanding, however, above description has been given about the case where the magnetic head is made by winding of a wire around a ferrite core (ferrite magnetic head).
On the other hand, a phenomenon observed only when a thin film magnetic head is used as the magnetic head is described below.
FIGS. 32(a) and (b) illustrates the phenomenon known as the negative edge. In this figure, (a) shows the received analog signal from a ferrite magnetic head, and (b) shows the received analog signal from a thin film magnetic head. The signals in (a) and (b) are received signals at magnetization reversals, and correspond to those expressed with broken lines in FIG. 26. The signal of FIG. 32 (a) is identical to those shown with broken lines in FIGS. 26, but does not contain any components with opposite polarity to the peak polarity of the received signal at the magnetization reversals. The signal of FIG. 32 (b) contains, unlike the signal of (a), components with opposite polarity to that of the received signal peaks (shaded parts). If the polarity of the received signal is positive, the polarity of the shaded parts is negative, and the shaded parts are called "negative edges."
When a magnetic head includes negative edges of the received analog signal, as in the case of a thin film magnetic head, the peak shift compensation should be performed taking the existence of negative edges into consideration.
For example, if all received signals at magnetization reversals expressed with broken lines in FIG. 26 have negative edges as shown in FIG. 32 (b), the received analog signal from the thin magnetic head obtained by integration of these signals would be extremely complicated. In addition, negative edges or overlapping negative edges might be detected as excessive peaks when the signal is differentiated.
When an equalizer with the construction of FIG. 28 is used, the received analog signal from the thin film magnetic head cannot be made sharper. The signal f(t+.tau.) contains negative edges, which are emphasized by the signals Kf(t) and Kf(t+2.tau.) shown in FIGS. 30 (a) and (b). Accordingly, the proper signal f'(t+.tau.) cannot be obtained. This may impede the peak shift compensation with an equalizer. A prior art invention proposal to solve the problem by eliminating negative edges is disclosed in the Japanese Patent Application Laid-open Print No. 61-99906. In this conventional method, the negative edge elimination and the peak shift compensation are performed in the reproducing process.